Keyhole stability in disc laser welding of AZ31B and AZ61A magnesium alloys and weld metal properties M. Wahba* 1,2 , M. Mizutani 3 , Y. Kawahito 3 and S. Katayama 3 To confirm the process potential and limitations of laser welding of magnesium alloys, a newly developed disc laser of 16 kW in maximum power was used to weld thin plates of AZ31B and AZ61A alloys. Melting characteristics and weld metal properties were studied under different process parameters. The very high power density of the applied beam caused the keyhole and the molten pool to be unstable. Underfill was the main welding defect limiting the process window and its formation was closely related to the keyhole instability. A process diagram indicating areas of sound and defected welds could be constructed. Sound welded joints could be obtained for AZ31B alloy with similar mechanical properties as that of the base material. The brittle intermetallics network present along the grain boundaries in AZ61A weld metals required a higher energy input to alleviate its detrimental effect on the mechanical properties of the welded joints. Keywords: Magnesium alloys, Laser welding, Keyhole, Welding defects, Underfill Introduction The growing pressure on the automotive industry to reduce vehicles CO 2 gas emissions and fuel consumption has made reduction of vehicles weight a key priority for manufacturers. Therefore, lightweight magnesium alloys are progressively utilised in construction. Other note- worthy characteristics are high specific strength, excel- lent machinability and castability, good damping behaviour and recyclability. 1,2 Significant implementa- tion of magnesium alloys into the vehicles structures necessitates the establishment of reliable welding methodologies. Compared with arc welding, laser beam welding (LBW) has many advantageous characteristics, such as high processing speed, low heat input, narrow fusion zone (FZ) and heat affected zone (HAZ), low distortion, deep penetration, and easy automation. 3,4 Moreover, the development of the new generation of diode pumped solid state lasers such as fibre and disc lasers has increased the capability and enhanced the performance of the LBW process. 5,6 Of particular interest is the enhanced beam quality which translates directly into higher welding speed, deeper penetration and longer distance to the workpiece. 7,8 Therefore, there has been a preferential tendency to replace arc welding of magne- sium alloys with LBW process. 9 Nevertheless, owing to the complexity of the physical phenomena in LBW process, some defects, which limit the processing window, are easily formed. 10–13 Understanding of these limiting factors is very essential to make full use of the potential of the LBW process. Most of the reported studies on LBW of wrought magnesium alloys 14–19 used the conventional CO 2 or Nd:YAG laser sources. It was concluded that LBW of magnesium alloys was characterised by a narrow operating window. However, there have been few attempts to study the limiting factors in the process. Although the formation of a keyhole is of fundamental importance for penetration welding, 3,4 the behaviour is hardly investigated. Therefore, a comprehensive study is needed to better understand the process and to investigate the potential of applying the new laser sources to welding of magnesium alloys. In the present study, an attempt was made to study the influence of different process parameters on keyhole stability and melting characteristics during disc laser welding of AZ31B and AZ61A magnesium alloys. Materials and experimental procedure The materials used are AZ31B and AZ61A magnesium alloy extruded sheets of 2?8 and 3 mm in thickness respectively. The chemical compositions of the alloys are given in Table 1. Rectangular test pieces with dimen- sions of 100650 mm were prepared by machining. Before welding, specimen surface was ground to remove the oxide layer and then cleansed with acetone. A schematic representation of the experimental set-up is shown in Fig. 1. Welding was performed by using a 16 kW continuous wave disc laser with a beam parameter product of 8 mm mrad. A laser beam was 1 Graduate School of Engineering, Osaka University, 1–1 Yamadaoka, Suita, Osaka 565 0871, Japan 2 Central Metallurgical Research and Development Institute, Helwan 11421, Egypt 3 Joining and Welding Research Institute, Osaka University, 11–1 Mihogaoka, Ibaraki, Osaka 567 0047, Japan *Corresponding author, email wahba@jwri.osaka-u.ac.jp ß 2010 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 9 June 2010; accepted 23 June 2010 DOI 10.1179/136217110X12720264008592 Science and Technology of Welding and Joining 2010 VOL 15 NO 7 559